I was re-reading the mounted combat rules and could not find a rule that explicitly states that a character needs the riding proficiency to cast while mounted in combat, only to attack. It only requires that the caster’s mount remain stationary. Obviously if the mount bolts or the caster is knocked from the mount the spell fails (not to mention the fact that even casters like to use melee once in a while), but the finicky nature of spell-casting made me expect stricter rules, so I wanted a clarification:
Can a character cast in combat while mounted without the riding proficiency?
If no, then can a caster cast while mounted if not in combat?
If yes, then how far away from unfriendly armed people constitutes “not in combat”?
What about casting from a mount when sharing a horse? For example: Say Reginald the mage hired on a henchmen who was a far better rider than he and bought a sturdy warhorse. Could Reginald cast when riding while his henchman steered (in this example the henchman has riding but the mage does not)? I know there are likely few if any military saddles meant to hold two riders on a horse, and taking such a group into melee is likely a bad idea, but a mage might like the idea of being able to run away faster.
On a related note: How strict is this “stationary” requirement? Specifically, if mounted on a flying mount attempting to hover, could a character cast? I would think that, even if a pegasus could hover in place, it would involve an irritating amount of up and down movement that, if anything, is more jarring than a smooth glide. This might be more complexity than is really necessary, but I could see a mount going into a gentle glide to try to make casting as easy as possible (of course, I’m assuming that the stationary requirement is to prevent jostling of the mage, not because the caster needs to stay still for other reasons such as a need to form magic in the air around themselves in a way that doesn’t work while moving quickly, etc.)